METHOD FOR HETEROEPITAXIAL GROWTH OF HIGH-QUALITY N-FACE GaN, InN, AND AlN AND THEIR ALLOYS BY METAL ORGANIC CHEMICAL VAPOR DEPOSITION

ABSTRACT

Methods for the heteroepitaxial growth of smooth, high quality films of N-face GaN film grown by MOCVD are disclosed. Use of a misoriented substrate and possibly nitridizing the substrate allow for the growth of smooth N-face GaN and other Group III nitride films as disclosed herein. The present invention also avoids the typical large (μm sized) hexagonal features which make N-face GaN material unacceptable for device applications. The present invention allows for the growth of smooth, high quality films which makes the development of N-face devices possible.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. Section 119(e) ofthe following co-pending and commonly-assigned U.S. patent application:U.S. Provisional Application Ser. No. 60/866,035 filed on Nov. 15, 2006,by Stacia Keller, Umesh K. Mishra and Nicholas A. Fichtenbaum, entitled“METHOD FOR HETEROEPITAXIAL GROWTH OF HIGH-QUALITY N-FACE GaN, InN, ANDAlN AND THEIR ALLOYS BY METAL ORGANIC CHEMICAL VAPOR DEPOSITION,”attorneys' docket number 30794.207-US-P1 (2007-121-1);

which application is incorporated by reference herein.

This application is related to the following co-pending andcommonly-assigned applications:

U.S. patent application Ser. No. 11/523,286, filed on Sep. 18, 2006, bySiddharth Rajan, Chang Soo Suh, James S. Speck, and Umesh K. Mishra,entitled “N-POLAR ALUMINUM GALLIUM NITRIDE/GALLIUM NITRIDEENHANCEMENT-MODE FIELD EFFECT TRANSISTOR”, attorney's docket number30794.148-US-U1 (2006-107-2), which claims priority to U.S. ProvisionalPatent Application Ser. No. 60/717,996, filed on Sep. 16, 2005, bySiddharth Rajan, Chang Soo Suh, James S. Speck, and Umesh K. Mishra,entitled “N-POLAR ALUMINUM GALLIUM NITRIDE/GALLIUM NITRIDEENHANCEMENT-MODE FIELD EFFECT TRANSISTOR”, attorney's docket number30794.148-US-P1 (2006-107-1); and

U.S. Provisional Patent Application Ser. No. 60/866,019, filed on Nov.15, 2006, by Nicholas A. Fichtenbaum, Umesh K. Mishra, and StaciaKeller, entitled “LIGHT EMITTING DIODE AND LASER DIODE USING N-FACE GaN,InN, and AlN AND THEIR ALLOYS”, attorney's docket number 30794.208-US-P1(2007-204-1);

which applications are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is related to growth of Group III nitride materials, andin particular to a method of heteroepitaxial growth of high quality,Nitrogen (N) face Gallium Nitride (GaN), Indium Nitride (InN), AluminumNitride (AlN), and their alloys, by Metal Organic Chemical VaporDeposition (MOCVD).

2. Description of the Related Art

(Note: This application references a number of different publications asindicated throughout the specification by one or more reference numberswithin brackets, e.g., [x]. A list of these different publicationsordered according to these reference numbers can be found below in thesection entitled “References.” Each of these publications isincorporated by reference herein.)

The use of group III nitride materials in consumer applications anddevices is becoming widespread. However, the majority of applicationsemploy Ga-polar group-III nitride films and heterostructures. Films andheterostructures of the opposite polarity (N-polar group-III nitridefilms and heterostructures) have been much less investigated due todifficulties in their growth. N-polar group-III nitride films andheterostructures are advantageous for the fabrication of a variety ofnitride based electronic and optoelectronic devices. The oppositedirection of the piezoelectric fields in N-polar heterostructures, incomparison to Ga-polar heterostructures, allows the fabrication oftransistor devices which cannot be fabricated using Ga-polarheterostructures.

One of the major challenges to III-Nitride based light emitters is thegrowth of high quality InGaN with high In composition. The use of theGallium (Ga)-face for devices limits the temperature at which the InGaNcan be grown, which limits the types of devices that can be made.Another challenge is the growth of low resistance p-type (Al,Ga,In)N:Mgfilms due to polarity conversion from Ga-face to N-face at high Mgdoping levels. By using N-face (Al,Ga,In)N layer structures both issuescan be mitigated.

The opposite direction of the piezoelectric fields in N-polar incomparison to Ga-polar heterostructures leads to a lower operatingvoltage of Light Emitting Diodes (LEDs) and Laser Diodes (LDs), and toimproved carrier injection in p-n junction devices, due to a narrowerwidth of the depletion region in general. Furthermore, the oppositedirection of the piezolelectric fields has advantages for devices suchas transistors, solar cells and devices utilizing tunnel junctions. Itcan be seen that there is a need in the art for N-face nitride materialsand methods to grow these materials.

SUMMARY OF THE INVENTION

The present invention describes growth of Group III nitride materials,and in particular a method of heteroepitaxial growth of high quality,N-face GaN, InN, AlN, and their alloys, and heterostructures comprisingthe same grown by MOCVD.

A method for growing an N-face group III nitride film in accordance withthe present invention comprises providing a substrate having a growthsurface with a misorientation angle between 0.5 and 10 degrees in anydirection relative to a miller indexed crystallographic plane [h, i, k,l] of the substrate, where h, i, k, l are miller indices; and growingthe N-face group III-nitride film on the growth surface, wherein thegroup III-nitride film having an N-face is smoother than an N-face groupIII-nitride film grown on a substrate without a misorientation angle.

If sapphire is used as substrate, such a method further optionallycomprises a misorientation of the [0001] sapphire with a direction of<11-20>, the miller indices of the misorientation direction are h=1,i=1, k=−2 and l=0 or equivalent. If the substrate is [000-1] C-facesilicon carbide, such a method optionally comprises a misorientationwith a misorientation direction of <1-100>, the miller indices of themirorientation direction are h=1, i=−1, k=0 and l=0 or equivalent.

Such a method can further optionally comprise the growing being by MetalOrganic Chemical Vapor Deposition (MOCVD), the group III-nitride layerhaving an N-face being grown on a nitridized misoriented substrate,forming an AlN layer on the misoriented substrate and growing the groupIII-nitride layer having an N-face on the AlN layer, the AlN layer beingdeposited using a step flow growth mode, the AlN layer setting anN-polarity for subsequently deposited group III-nitride layers, themisoriented substrate being a sapphire substrate, depositing a groupIII-nitride nucleation layer on an AlN layer formed on the sapphiresubstrate due to the nitridization, and growing the main groupIII-nitride layer on the group III-nitride nucleation layer, the groupIII-nitride nucleation layer being deposited using a step flow growthmode or layer by layer growth mode, the group III-nitride nucleationlayer being at least partially doped, the growing of the groupIII-nitride layer having an N-face comprises doping and growing, on thenucleation layer, a first group III nitride layer having an N-face,using a low ammonia partial pressure and growing a second groupIII-nitride layer having an N-face, on the first group-III layer havingan N-face, at a high ammonia partial pressure, so that at least part ofthe second group III-nitride layer having an N-face is doped, themisoriented substrate being a polished Carbon Polar Silicon Carbidesubstrate, and depositing a graded or stepped group III-nitride layer,e.g., the layer has a changing composition of one of the elements usedto make up the layer, on the AlN layer, wherein an Al composition of thegraded or stepped group III-nitride layer is graded or stepped from AlNto GaN.

Additional optional items in accordance with the present inventioninclude the graded group III-nitride layer is at least partially doped,the growing of the group III-nitride layer having an N-face comprisesdoping and growing, on the graded group III-nitride layer, a first groupIII-nitride layer having an N-face, using a low ammonia partialpressure, and growing a second group III-nitride layer having an N-face,at a high ammonia partial pressure, so that at least part of the secondgroup III-nitride layer having an N-face is doped, and a devicefabricated using the method.

Another method for creating a group III-nitride film with an abruptp-type doping profile in accordance with the present invention comprisesproviding a substrate having a growth surface with a misorientationangle between 0.5 and 10 degrees relative to a miller indexedcrystallographic plane [h, i, k, l] of the substrate, where h, i, k, lare miller indices; and growing the N-face group III-nitride film havingan abrupt p-type doping profile on the growth surface, wherein the groupIII-nitride film having an N-face is smoother than an N-face groupIII-nitride film grown on a substrate without a misorientation angle.

Another method for enhancing charge transport properties of a nitridedevice in accordance with the present invention comprises fabricatingthe nitride device using N-face nitride layers grown on a substratehaving a growth surface with a misorientation angle between 0.5 and 10degrees relative to a miller indexed crystallographic plane [h, i, k, l]of the substrate, where h, i, k, l are miller indices, and aligning achannel of the nitride device substantially perpendicular to amisorientation direction of the misoriented (Al, Ga, In)N epitaxiallayer, wherein charge transport properties are enhanced perpendicular tothe misorientation direction.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 illustrates a process chart in accordance with the presentinvention, for the growth on sapphire.

FIG. 2A illustrates a process chart in accordance with the presentinvention, for the growth on silicon carbide.

FIG. 2B illustrates a process chart in accordance with the presentinvention, for the growth on silicon.

FIG. 3 illustrates a process chart in accordance with the presentinvention, for the growth of semi-insulating GaN on sapphire.

FIG. 4A illustrates a process chart in accordance with the presentinvention, for the growth of semi-insulating GaN on silicon carbidesubstrate.

FIG. 4B illustrates a process chart in accordance with the presentinvention, for the growth of semi-insulating GaN on a silicon (111)substrate.

FIG. 5( a) shows an optical microscope image of an N-face GaN film grownby MOCVD on a nitrided sapphire substrate, and FIG. 5( b) shows anatomic force microscope (AFM) image of N-face GaN grown by MOCVD on anitrided sapphire substrate

FIG. 6 shows transmission electron micrographs, under different imagingconditions, of an N-face GaN film grown using the present invention FIG.7( a) shows an AFM image of InGaN/GaN multi quantum wells (MQWs) grownusing the present invention, and FIG. 7( b) shows photoluminescence (PL)from MQWs comprising 5×(3 nm thick In_(0.8)Ga_(0.9)N/8 nm thick GaN).

FIG. 8 shows X-ray Diffraction (XRD) of N-face nitride MQWs comprising5×(4 nm thick In_(0.12)Ga_(0.88)N/10 nm thick GaN).

FIG. 9 is a graph showing SIMS results of oxygen impurities as afunction of temperature.

FIG. 10 is a graph showing SIMS results of carbon impurities as afunction of temperature.

FIG. 11( a) and 11(b) are graphs of SIMS results of Mg incorporation inan N-face (FIG. 11( a)) and a Ga-face (FIG. 11( b)) GaN SIMS stack.

FIGS. 12 a-12 f illustrate optical micrographs of 0.8 micron thick GaNfilms grown on sapphire substrates in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Overview

This invention will allow for the creation of transistors that cannot befabricated on traditional Ga-polar GaN. The present invention will alsoallow novel polarization-based band structure designs to create moreefficient LEDs and LDs. The reverse polarization fields in N-polarheterostructures, in comparison to Ga-polar heterostructures, result inlower operating voltages, shrink the width of the depletion region ofthe p-n-junction, and improve carrier injection, leading to advances indevice performance. Deposition on N-polar surfaces enables the growth ofInGaN with higher compositions of Indium compared to Ga-polar surfaces,easing the push of LED wavelengths into the yellow and red portions ofthe color spectrum. Additionally, the present invention enables the useof higher p-type doping levels in GaN based devices, leading to improveddevice performance. Further, novel electronic and optoelectronic nitridedevices such as N-face transistors (HEMTs), N-face LEDs, and N-face LDs,will be possible using the materials grown using the present invention.

This invention enables the heteroepitaxial growth of high quality,smooth N-face (000-1) InN, GaN, AlN films, and their alloys, by MOCVD.Since free standing GaN substrates are not readily available andaffordable, GaN is usually grown heteroepitaxially on Silicon Carbide(SiC), Sapphire, Si, and various other substrates.

The group III-Nitrides have large built-in polarization fields centeredabout the direction which makes the direction, and hence face of growth,an extremely important parameter in the growth and design of devices.Traditionally all III-Nitride growth is performed on the (0001)III-face. Since this invention allows N-face growth, new transistor(HEMTs), LEDs and LDs designs will be possible through the differentphysical properties provided by N-face. For HEMTs, the (000-1) N-faceallows, for example, the design of structures with extremely low gateleakages for high power, high frequency device operation. For lightemitters (LEDs, and LDs) the N-face enables the growth of betterquality, high Indium composition InGaN alloys, which are currentlyneeded to create high power devices in the green, yellow, and red partsof the color spectrum. Additionally, traditional Ga-face GaN suffersfrom inversion, while N-face GaN does not, when doped highly with Mg,which is need to create p-type GaN. Higher p-type carrier concentrationsshould drastically increase device performances. Moreover, the reversepolarization fields in N-face in comparison to Ga-face heterostructureslower the operating voltage of LEDs and LDs. This also results in anarrower width of the depletion region of the p-n-junction and improvedcarrier injection leading to advances in the device performance.

Whereas smooth N-face GaN films can be easily obtained on C-face SiCsubstrates using Molecular Beam Epitaxy (MBE) as growth techniques,films grown by MOCVD typically exhibit rough surfaces caused by theformation of hexagonal hillocks, independent of whether the GaN layerswere grown on small GaN single crystals or on foreign substrates,hampering the development of N-face devices. [1, 2, 3] However,investigations on GaN single crystals showed that the hillock formationcan be largely suppressed if the MOCVD growth is performed on miscut GaNcrystals [2,3]. More recently, smooth N-face GaN films were alsoobtained on sapphire substrates by Matsuoka et al. [4].

The present invention utilizes a misoriented substrate, for example(0001) sapphire or (000-1) SiC, which are misoriented for example in the<1-100> or <11-20> direction in order to obtain smooth N-face filmsgrown by MOCVD. Thereby the technique can be applied to any foreignsubstrate, e.g., sapphire, SiC or Si.

Thereby, for growth on sapphire, the preferred substrate misorientationdirection is <11-20>, leading to group-III nitride surface steps and/orundulations parallel to the <11-20> direction of the GaN crystal. Forgrowth on carbon-polar silicon carbide, the preferred substratemisorientation direction is <1-100>, also leading to group-III nitridesurface steps and/or undulations parallel to the <11-20> direction ofthe GaN crystal.

The preferred misorientation angle ranges between 1-5 degrees relativeto a miller indexed surface [h, i, k, l] of the substrate, where h, i,k, l are miller indices. All misorientation angles and directions aregiven with respect of either the (0001) (=c-plane) sapphire surface(=plane) or the (000-1) SiC substrate surface. Both are suitable for thegrowth of (000-1) GaN. The present invention can also work for thegrowth of other N-terminated GaN planes on other substrate planes.

Technical Description

Growth on Sapphire

For growth on sapphire, the present invention is realized by:

a. Placing a misoriented sapphire substrate in the MOCVD reactor andannealing at around 1090° C. surface temperature in a H₂ environment.

b. The substrate is then nitridized in NH₃ and H₂ for several seconds atapproximately 980° C. surface temperature, leading to the formation of athin AlN layer on the sapphire which sets the N-face polarity of thegrowth.

c. NH₃ and Trimethylgallium (TMG) are then flowed into the reactor tocommence growth of GaN on the AlN. Thereby, first an about 20 nm thickGaN layer can be deposited at medium growth temperatures (surfacetemperature 950° C.), leading to growth in a step-flow or layer-by-layergrowth mode.

d. NH₃ and Trimethylgallium (TMG) are then flowed into the reactor tocommence growth of the main GaN on the layer formed in step (c).

Currently, the growth method being used is MOCVD; however this inventioncould become quite useful for other growth methods such as MBE, HydrideVapor Phase Epitaxy (HVPE), and Chemical Beam Epitaxy (CBE), etc.

The process described in part (c) applies very generally to GaN growth.However, the invention can be easily utilized to create InN (with theuse of TMI), AlN (with the use of TMA), or any of their alloys, alsowith other group-V atoms (Al,Ga,In)(N,P,As), by combining theappropriate precursors.

Other precursors can be used such as triethylgallium, triethylaluminumfor group-III sources, and dimethylhydrazine for N sources. Anyappropriate substrate can be used, such as SiC, Si, spinel, etc. Thenitridation temperature and time can be varied, or completelyeliminated, dependent on the substrate. Other growth initiationprocedures can be applied. For example, the growth can be initiated withthe deposition of an AlN layer etc., or no growth initiation step mightbe required at all. Note that, while part (b) specifies the depositionof a thin AlN layer, any thickness of the AlN layer may be used, andnitride materials other than AlN may be used. The annealing of thesubstrate can be eliminated. The substrate can be misoriented not onlytowards the <11-20> and <1-100> directions, but also in any otherdirection. Instead of a single composition layer, the composition of thelayer/crystal can be modified for all individual growth steps. Thegrowth of each individual layer may also be interrupted for thedeposition of layers comprised of other materials, for example siliconnitride or silicon oxide, possibly for strain or dislocation management.

FIG. 1 illustrates a process chart in accordance with the presentinvention.

Block 100 represents the step of placing a misoriented sapphiresubstrate in a MOCVD chamber.

Block 102 represents the step of annealing the misoriented sapphiresubstrate.

Block 104 represents the step of nitridizing the annealed misorientedsapphire substrate to form a thin AlN surface layer to set an N-polarityfor following group III nitride layers. [7,8] Reference 7 describes theformation of the AlN layer, reference 8 is a theoretical calculationwith the result that the AlN layer should be one bi-layer thick,corresponding to ˜0.5 nm.

Block 106 represents the step of depositing, on the AlN surface layer, agroup III nitride nucleation layer in a step-flow or layer-by-layergrowth mode.

Block 108 represents the step of growing the main group III nitridelayer on the nucleation layer.

Growth on Carbon Polar SiC

For growth on carbon (C) polar SiC, this invention is realized by:

a. Placing a misoriented, polished (e.g. chemo-mechanically polished)C-polar SiC substrate in the MOCVD reactor and annealing at around 1090°C. surface temperature in a H₂ environment.

b. NH₃ and Trimethylaluminum (TMA) are then flowed into the reactor tocommence growth of a thin AlN layer in a step-flow-growth-mode orlayer-by-layer growth mode. To increase the surface mobility ofAl-species, a surfactant, for example indium, in form of trimethylindium(TMI), can be additionally flowed into the reactor [5].

c. Trimethylgallium (TMG) is then also optionally flowed into thereactor to commence the growth of a graded or stepped Al_(x)Ga_(1-x)Nlayer, the composition of which is graded or stepped from AlN to GaN.The injection of TMI can be continued to ensure a step-flow-growth-modeor layer-by-layer growth mode of the layer.

d. NH₃ and Trimethylgallium are then flowed into the reactor to commencegrowth of the main GaN layer.

Currently, the growth method being used is MOCVD; however this inventioncould become quite useful for other growth methods such as MBE, HVPE,CBE, etc.

The process described in part (c) applies very generally to GaN growthhowever, the invention can be easily utilized to create InN (with theuse of TMI), AlN (with the use of TMA), or any of their alloys, alsowith other group-V atoms (Al, Ga, In)(N, P, As), by combining theappropriate precursors. Other precursors can be used such astriethylgallium, triethylaluminum for group-III sources, anddimethylhydrazine for N sources. Any appropriate substrate can be used,such as SiC, Si, spinel, etc. The nitridation temperature and time canbe varied, or completely eliminated, dependent on the substrate. Othergrowth initiation procedures can be applied. For example, the growth canbe initiated with the deposition of an AlGaN layer etc., or no growthinitiation step might be required at all. Note that, while part (b)specifies the deposition of a thin AlN layer, any thickness of the AlNlayer may be used, and nitride materials other than AlN may be used. Theannealing step of the substrate can be eliminated. The graded or steppedAlGaN layer can be eliminated and be replaced by any (Al, Ga, In)N layerof any composition if grown in a step-flow-growth-mode orlayer-by-layer-growth-mode. The substrate can be misoriented not onlytowards the <1-100> or <11-20> directions but also in any otherdirection. Instead of a single composition layer, the composition of thelayer/crystal can be modified for all individual growth steps. Thegrowth of each individual layer may also be interrupted for thedeposition of layers comprised of other materials, for example siliconnitride or silicon oxide, possibly for strain or dislocation management.

FIG. 2A illustrates a process chart in accordance with the presentinvention.

Block 200 represents the step of placing a misoriented,chemo-mechanically polished C-polar SiC substrate in a MOCVD chamber.

Block 202 represents the step of annealing the misoriented SiCsubstrate.

Block 204 represents the step of depositing, on the annealed misorientedsubstrate, a thin AlN layer in a step-flow or layer-by-layer growthmode.

Block 206 represents the optional step of introducing an additionalGa-precursor into the MOCVD chamber to deposit, on the AlN, a layerwhere the Al-composition is graded or stepped from AlN to GaN in astep-flow or layer by layer growth mode.

Block 208 represents the step of growing the main group III nitridelayer on the graded layer.

Growth on Si

For growth on Si (111), this invention is realized by:

Placing a misoriented, polished (e.g. chemo-mechanically polished) Si(111) substrate in the MOCVD reactor and annealing at around 1090° C.surface temperature in a H₂ environment.

The substrate is then nitridized in NH₃ and H₂ for several seconds atapproximately 980° C. surface temperature, leading to the formation of athin silicon nitride surface layer on the sapphire which sets the N-facepolarity of the growth.

NH₃ and Trimethylaluminum (TMA) are then flowed into the reactor tocommence growth of a thin AlN layer in a step-flow-growth-mode orlayer-by-layer growth mode. To increase the surface mobility ofAl-species, a surfactant, for example indium, in form of trimethylindium(TMI), can be additionally flowed into the reactor [5].

Trimethylgallium (TMG) is then also flowed into the reactor to commencethe growth of a graded or stepped Al_(x)Ga_(1-x)N layer, the compositionof which is graded or stepped from AlN to GaN. The injection of TMI canbe continued. (step optional)

NH₃ and Trimethylgallium are then flowed into the reactor to commencegrowth of the main GaN layer.

Currently, the growth method being used is MOCVD; however this inventioncould become quite useful for other growth methods such as MBE, HVPE,CBE, etc.

The process described applies very generally to GaN growth however, theinvention can be easily utilized to create InN (with the use of TMIinstead of TMG), AlN (with the use of TMA instead of TMG), or any oftheir alloys, also with other group-V atoms (Al, Ga, In)(N, P, As), bycombining the appropriate precursors. Other precursors can be used suchas triethylgallium, triethylaluminum for group-III sources, anddimethylhydrazine for N sources. Any appropriate substrate can be used,such as SiC, Si, spinel, etc. The nitridation temperature and time canbe varied. Other growth initiation procedures can be applied. Forexample, the growth can be initiated with the deposition of an AlGaNlayer etc., or no growth initiation step might be required at all. Notethat, while part (b) specifies the deposition of a thin AlN layer, anythickness of the AlN layer may be used, and nitride materials other thanAlN may be used. The annealing step of the substrate can be eliminated.The graded or stepped AlGaN layer can be eliminated and be replaced byany (Al,Ga,In)N layer of any composition if grown in astep-flow-growth-mode or layer-by-layer-growth-mode. The substrate canbe misoriented not only towards the Si <−110> or Si <11-2> directionsbut also in any other direction. Misoriented Si (001) instead of Si(111)can be used as substrate. Instead of a single composition layer, thecomposition of the layer/crystal can be modified for all individualgrowth steps. The growth of each individual layer may also beinterrupted for the deposition of layers comprised of other materials,for example silicon nitride or silicon oxide, possibly for strain ordislocation management.

FIG. 2B illustrates a process chart in accordance with the presentinvention.

Block 210 represents the step of placing a misoriented,chemo-mechanically polished (111) Si substrate in a MOCVD chamber.

Block 212 represents the step of annealing the misoriented Si substrate.

Block 214 represents the step of nitridizing the misoriented Sisubstrate.

Block 216 represents the step of depositing, on the nitridizedmisoriented substrate, a thin AlN layer in a step-flow or layer-by-layergrowth mode.

Block 218 represents the optional step of introducing an additionalGa-precursor into the MOCVD chamber to deposit, on the AlN, a layerwhere the Al-composition is graded or stepped from AlN to GaN in astep-flow or layer by layer growth mode.

Block 220 represents the step of growing the main group III nitridelayer on the graded layer.

Growth of Semi-Insulating Group-III Nitride Layers

For the growth of semi-insulating group-III nitride base layers a dopantwith acceptor character, for example iron, possibly using the precursorbis-cyclopentadienyl-iron (Cp)₂Fe, is added during the growth of thelayers or part of the layer growth, as seen in the process charts ofFIGS. 3 and 4.

Devices utilizing lateral conduction, such as transistors, fabricatedfrom group-III nitride heterostructures grown using this invention shallbe aligned in such a way that the lateral carrier conduction occursparallel to surface steps/undulations of the group-III nitride crystal.For example for layer structures deposited on sapphire with amisorientation direction of <11-20>, or on C-polar SiC substrates with amisorientation direction of <1-100>, both leading to group-III nitridesurface steps/undulations parallel to the <11-20> direction of the GaNcrystal, source and drain contacts of transistors need to be aligned insuch a way that the transistor channel is aligned parallel to the<11-20> direction of the GaN crystal.

FIG. 3 illustrates a process chart in accordance with the presentinvention for the growth of semi-insulating GaN on sapphire.

Block 300 represents the step of placing a misoriented sapphiresubstrate in a MOCVD chamber.

Block 302 represents the step of annealing the misoriented sapphiresubstrate.

Block 304 represents the step of nitridizing the annealed misorientedsapphire substrate to form a thin (AlN surface layer) to set anN-polarity for following group-III nitride layers.

Block 306 represents the step of depositing, on the AlN, a group-IIInitride nucleation layer in a step-flow or layer-by-layer growth mode,where either the entire layer or a part of the nucleation layer is dopedwith iron.

Block 308 represents the step of growing, on the AlN layer, the maingroup-III nitride layer, where either the entire layer or a part of thelayer is doped with iron (the second group-III nitride layer having anN-face).

FIG. 4A illustrates a process chart in accordance with the presentinvention for the growth of semi-insulating GaN on a SiC substrate.

Block 400 represents the step of placing a misoriented,chemo-mechanically polished C-polar SiC substrate in a MOCVD chamber.

Block 402 represents the step of annealing the misoriented SiCsubstrate.

Block 404 represents the step of depositing, on the annealed substrate,a thin AlN layer in a step-flow or layer-by-layer growth mode whereeither the entire layer or a part of the layer is doped with iron.

Block 406 represents the optional step of introducing an additionalGa-precursor into the MOCVD chamber to deposit, on the AlN, a layerwhere the Al-composition is graded or stepped from AlN to GaN in astep-flow or layer by layer growth mode, where either the entire layeror a part of the layer is doped with iron.

Block 408 represents the step of growing, on the graded layer, the maingroup-III nitride layer where the entire layer or part of the layer aredoped with iron (the group-III nitride layer having an N-face).

FIG. 4B illustrates a process chart in accordance with the presentinvention for the growth a Si (111) substrate.

Block 410 represents the step of placing a misoriented,chemo-mechanically polished Si substrate in a MOCVD chamber.

Block 412 represents the step of annealing the misoriented Si substrate.

Block 414 represents the step of nitridizing the misoriented Sisubstrate.

Block 416 represents the step of depositing, on the annealed substrate,a thin AlN layer in a step-flow or layer-by-layer growth mode whereeither the entire layer or a part of the layer is doped with iron.

Block 418 represents the optional step of introducing an additionalGa-precursor into the MOCVD chamber to deposit, on the AlN, a layerwhere the Al-composition is graded or stepped from AlN to GaN in astep-flow or layer by layer growth mode, where either the entire layeror a part of the layer is doped with iron.

Block 420 represents the step of growing, on the graded layer, the maingroup-III nitride layer where the entire layer or part of the layer aredoped with iron (the first Fgroup-III nitride layer having an N-face).

While the present invention, in FIGS. 3 to 4 for example, describes thegrowth of main group III-nitride layer(s), any nitride layers having anN-face may be grown, for example group III-V layers or group III-nitridelayers having an N-face, as discussed throughout the specification.

Fe dopants can be replaced by other dopants with acceptor character, forexample Mg, Zn or C.

The heteroepitaxially N-polar group-III nitride films grown according tothe present invention serve as base layers for following group-IIInitride layer sequences according to the specific device application.

FIG. 5( a) shows an optical microscope image of an N-face GaN film grownby MOCVD on a nitridized sapphire substrate. FIG. 5( b) shows an atomicforce microscope (AFM) image of N-face GaN grown by MOCVD on anitridized sapphire substrate, wherein the root mean square (RMS)roughness is 0.9 nm.

FIG. 6 shows transmission electron micrographs, under different imagingconditions, of an N-face GaN film grown using the present invention. Theestimated threading dislocation density is at most of the order 10⁹cm⁻².

FIG. 7( a) shows an AFM image of InGaN/GaN multi quantum wells (MQWs)grown using the present invention, where the RMS roughness is 0.85 nm,showing a smooth surface at InGaN growth temperatures. FIG. 7( b) shows300 Kelvin photoluminescence (PL) of MQWs comprising 5×(3 nm thickIn_(0.1)Ga_(0.9)N/8 nm thick GaN), grown by MOCVD according to thepresent invention.

FIG. 8 shows X-ray Diffraction (XRD) of N-face nitride MQWs comprising5×(4 nm thick In_(0.12)Ga_(0.88)N/10 nm thick GaN), which is comparableto Ga-face XRD results.

Growth of P-Type N-Polar Group-III Nitrides

Similar to Ga-polar group-III nitrides, p-type doping can be executedusing bis-cyclopentadienyl magnesium or one of its derivates asprecursor. In N-polar nitride films, however, higher Mg doping levelscan be realized without degradation of the crystal quality and thesurface morphology. In addition, sharper Mg-doping profiles can berealized, realizing great advantages for a variety of p-n junctiondevices.

Stabilization of the Surface

To stabilize the surface of any N-polar group-III nitride film, a thininsulator layer, for example silicon nitride or silicon oxide, can bedeposited on top of the nitride film, possibly in-situ. The surface canalso be stabilized through the deposition of a thin p-type N-polarnitride film, which can be fabricated through doping with Mg using theprecursor bis-cyclopentadieny-magnesium, for example.

Growth of Group-III Nitrides with Surface Planes Comprised of a HighFraction of N Atoms

The use of misoriented substrates is beneficial also for the growth ofgroup-III nitride films with surfaces which are comprised of a highfraction of N atoms other than the (000-1) surface, for examplesemi-polar N-face films.

Impurity Incorporation in Heteroepitaxial N-face and Ga-face GaN Grownby MOCVD.

In group III-nitrides, the crystal growth orientation has a substantialimpact upon the chemical and physical properties of the material. Amongthese properties is the impurity incorporation, which has been studiedon Ga-face (0001) GaN, but has not been extensively explored for N-face(000-1) GaN. Presumably, the discrepancy in the understanding betweenthe two polarities arises from the historically rough hexagonal surfacemorphology of N-face GaN when grown by MOCVD. However, the presentinvention has shown that through the use of vicinal sapphire substrates,smooth N-face GaN can be grown heteroepitaxially by MOCVD. As such,surface roughnesses and threading dislocations grown on misorientedsubstrates, as shown in the present invention, are comparable to Ga-faceGaN films.

The present invention has also studied the difference in impurityincorporation between MOCVD grown N-face GaN on different sapphireoff-cuts, and Ga-face GaN, using secondary ion mass spectroscopy (SIMS).The unintentional impurities oxygen, carbon, and hydrogen, as well asthe intentional impurities silicon and magnesium, were studied as afunction of changes in the temperature, pressure, V/III ratio and Gaflow.

Ga-face and N-face GaN templates, approximately 1 μm thick, were firstgrown separately, due to the need for different growth initiationconditions. For the N-face, templates were grown on off-cuts of 2°, 4°,5° towards the sapphire [10-10] direction as well as 4° and 5° towardsthe sapphire [11-20] direction. A piece of the Ga-face and each of theN-face templates were then co-loaded in the MOCVD reactor, where a “SIMSstack” was regrown which allowed a direct comparison between each of theoff-cuts and polarities. The first SIMS stack explored variations in Gaflow and pressure, while the second contained variations in temperatureand V/III ratio.

The SIMS results for oxygen incorporation as a function of temperature,shown in FIG. 9, indicated that oxygen incorporation on all of theN-face off-cuts was substantially higher than the Ga-face. However, thecarbon incorporation as a function of temperature, shown in FIG. 10, wassubstantially higher on the Ga-face when compared to all of the N-faceoff-cuts. Models based upon the difference in atomic bonding onN-/Ga-face surfaces will be presented elsewhere. Additionally, theimpact of changes in the growth conditions for oxygen and carbonincorporation will be discussed elsewhere. Hydrogen incorporation wascomparable between both polarities. The present invention found that Mgand Si incorporation was also comparable on all samples.

However, the present invention found a significant difference in the Mgincorporation profiles, between N- and Ga-face samples, as shown in FIG.11. These results present a direct comparison between the impurityincorporation of Ga- and N-face GaN films, and indicate that impurityincorporation on smooth N-face can be controlled through growthconditions for use in device applications. P-type (e.g. Mg) dopingN-face nitride films leads to the creation of abrupt p-type dopingprofiles, as evidenced by the profile 1100 in FIG. 11( a), as comparedto the less well defined p-type doping profiles 1102 produced whenp-type doping Ga-face nitride films. Thus, the present inventionprovides a method for the creating a group III-nitride film with anabrupt p-type doping profile, comprising growing and doping a groupIII-nitride film with an N-face.

Advantages and Improvements

Currently most N-polar GaN films grown by MOCVD, which is the mostcommonly used growth method for large scale fabrication of GaN baseddevices, are characterized by large (μm sized) hexagonal features whichmake the material unacceptable for device applications. This inventionallows for the growth of smooth, high quality films which makes thedevelopment of N-face devices possible.

For HEMTs, device structures which were not feasible with thetraditional Ga-face will now be available with smooth N-face growth.

One of the major challenges to III-Nitride based light emitters is thegrowth of high quality InGaN. N-face allows the growth of InGaN athigher temperatures than the traditional Ga-face, which provides betterquality material as well as making higher indium content films feasible[6].

Another challenge to the growth of light emitters is p-type doping. Inthe traditional Ga-face material, too high p-type doping (Mg) causes thesurface to locally invert to N-face causing a poor quality film. Sincethe growth is now performed on the N-face the film quality can bemaintained at higher levels of p-type doping which will lead to a muchbetter device performance. In addition sharper Mg-doping profiles can berealized further improving the device performance.

III-Nitride based light emitters suffer from strong polarization inducedelectric fields. N-face material provides an electric field in theopposite direction to the traditional Ga-face which should allow, forinstance, for lower operating voltages and improved carrier injectionresulting in an increased efficiency in light emitting devices.

The etching properties of N-face are distinctly different from that ofGa-face which will be useful in creating better light extraction schemesin LEDs, such as surface roughening, and mega-cones, as well as etchedfacets for LDs.

The Mg memory effect, present on Ga-face, is not significant on theN-face, which allows for the creation of abrupt p-type doping profiles,not available on the Ga-face, for use in device structures.

N-polar devices grown on misoriented substrates make use of enhancedcharge (e.g electron and/or hole) transport properties in a specificdirection related to the misorientation direction through appropriatealignment of the device channel with respect to the misorientationdirection. So, for example, and not by way of limitation, the channel ofa transistor or charge transport channel of any device can be createdsuch that the orientation of the channel and the enhanced chargetransport properties are both used to create the desired chargetransport for a given device. Some devices may desire faster chargetransport, and, as such, the channel would be aligned perpendicular tothe misorientation direction of the N-face (Al,Ga,In)N layer(s) toincrease charge transport in such devices; other devices may require aresistance or other slowing of the charge transport, and the channel canbe aligned parallel or at some other alignment other than parallel tothe enhanced charge transport of the misoriented N-face film grown onthe misoriented substrate. Such design characteristics can now be takeninto account when designing the device which were previously unavailableto the device designer.

FIGS. 12 a-12 f illustrate optical micrographs of 0.8 micron thick GaNfilms grown on sapphire substrates in accordance with the presentinvention.

FIG. 12 a shows growth in a misorientation of 0.5 degrees towards thea-plane, and

FIG. 12 b shows growth in a misorientation of 0.5 degrees toward them-plane.

FIG. 12 c shows growth in a misorientation of 1 degree towards thea-plane, and FIG. 12 d shows growth in a misorientation of 1 degreetoward the m-plane.

FIG. 12 e shows growth in a misorientation of 2 degrees towards thea-plane, and FIG. 12 f shows growth in a misorientation of 2 degreestoward the m-plane.

The inserts in FIGS. 12 a and 12 b are enlarged 3 times from that of themain figures.

REFERENCES

The following references are incorporated by reference herein:

-   [1] Homo-epitaxial GaN growth on exact and misoriented single    crystals: suppression of hillock formation: A. R. A. Zauner, J. L.    Weyher, M. Plomp, V. Kirilyuk, I. Grzegory, W. J. P. van    Enckevort, J. J. Schermer, P. R. Hageman, and P. K. Larsen, J.    Cryst. Growth 210 (2000) 435-443.-   [2] Homo-epitaxial GaN growth on the N-face of GaN single crystals:    the influence of the misorientation on the surface    morphology: A. R. A. Zauner, A. Aret, W. J. P. van Enckevort, J. L.    Weyher, S. Porowski, J. J. Schermer, J. Cryst. Growth 240 (2002)    14-21.-   [3] A. P. Grzegorczyk et al. Influence of sapphire annealing in    trimethylgallium atmosphere on GaN epitaxy by MOCVD: J. Cryst.    Growth 283 (2005) 72-80.-   [4] N-polarity GaN on sapphire substrates grown by MOCVD: T.    Matsuoka, Y. Kobayashi, H. Takahata, T. Mitate, S Mizuno, A.    Sasaki, M. Yoshimoto, T. Ohnishi, and M. Sumiya, Phys. Stat.    Sol. (b) 243 (2006) 1446-1450.-   [5] Indium-surfactant-assisted growth of high-mobility AlN/GaN    multilayer structures by MOCVD, S. Keller, S. Heikman, I.    Ben-Yaakov, L. Shen, S. P. DenBaars, and U. K. Mishra, Appl. Phys.    Lett. 79 (2001) 3449.-   [6] The effect of substrate polarity on the growth of InN by RF-MBE:    Naoi et al., J. Cryst. Growth 269 (2004) 155-161.-   [7] Nitridation of sapphire. Effect on the optical properties of GaN    epitaxial overlayers: N. Grandjean, J. Massies, and M. Leroux, Appl.    Phys. Lett. 69 (1996) 2071.-   [8] Energetics of AlN thin films on the Al₂O₃ (0001) surface: R. Di    Felice and J. Northrup, Appl. Phys. Lett. 73 (1998) 936.

CONCLUSION

This concludes the description of the preferred embodiment of thepresent invention. The foregoing description of one or more embodimentsof the invention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto and the full rangeof equivalents to the claims appended hereto.

1. A method for growing an N-face group III nitride film, comprising:(a) providing a substrate having a growth surface with a misorientationangle between 0.5 and 10 degrees relative to a miller indexedcrystallographic plane [h, i, k, l] of the substrate, where h, i, k, lare miller indices; and (b) growing the N-face group III-nitride film onthe growth surface, wherein the N-face group III-nitride film issmoother than an N-face group III-nitride film grown on a substratewithout a misorientation angle.
 2. The method of claim 1, wherein themiller indices are h=1, i=0, k=0 and l=−1 and the substrate is siliconcarbide.
 3. The method of claim 1, wherein the miller indices are h=0,i=0, k=0 and l=1 and the substrate is sapphire.
 4. The method of claim1, wherein the miller indices are h=1, k=1, l=1 and the substrate issilicon.
 5. The method of claim 1, wherein the substrate is (001) Si. 6.The method of claim 1, wherein the growing is by Metal Organic ChemicalVapor Deposition (MOCVD).
 7. The method of claim 1, wherein the N-facegroup III-nitride film is grown on a nitridized misoriented substrate.8. The method of claim 1, further comprising forming an AlN layer on themisoriented substrate and growing the N-face group III-nitride film onthe AlN layer.
 9. The method of claim 8, wherein a (Al,Ga,In)Nnucleation layer is deposited onto the AlN layer using a step flowgrowth mode.
 10. The method of claim 8, wherein the AlN layer sets anN-polarity for subsequently deposited group III-nitride layers.
 11. Themethod of claim 7, wherein the misoriented substrate is a sapphiresubstrate.
 12. The method of claim 11, further comprising depositing agroup III-nitride nucleation layer on an AlN layer formed on thesapphire substrate due to the nitridization, and growing the N-facegroup III-nitride film on the group III-nitride nucleation layer. 13.The method of claim 12, wherein the group III-nitride nucleation layeris deposited using a step flow growth mode.
 14. The method of claim 12,wherein the group III-nitride nucleation layer is at least partiallydoped.
 15. The method of claim 14, wherein the growing of the N-facegroup III-nitride film comprises: (a) doping and growing, on thenucleation layer, a first N-face group III nitride layer, and (b)growing a second N-face group III-nitride layer on the first N-facegroup-III layer so that at least part of the second N-face groupIII-nitride layer is doped.
 16. The method of claim 8, wherein themisoriented substrate is a polished Carbon Polar Silicon Carbidesubstrate.
 17. The method of claim 16, further comprising depositing agroup III-nitride layer having a changing composition on the AlN layer,wherein an Al composition of the group III-nitride layer having thechanging composition is graded from AlN to GaN.
 18. The method of claim17, wherein the graded group III-nitride layer is at least partiallydoped.
 19. The method of claim 18, wherein the growing of the groupIII-nitride layer having an N-face comprises: (a) doping and growing, onthe graded group III-nitride layer, a first group III-nitride layerhaving an N-face and (b) growing a second group III-nitride layer havingan N-face, so that at least part of the second group III-nitride layerhaving an N-face is doped.
 20. A device fabricated using the method ofclaim
 1. 21. A method for creating a group III-nitride film with anabrupt p-type doping profile, comprising (a) providing a substratehaving a growth surface with a misorientation angle between 0.5 and 10degrees relative to a miller indexed crystallographic plane [h, i, k, l]of the substrate, where h, i, k, l are miller indices; and (b) growingthe N-face group III-nitride film having an abrupt p-type doping profileon the growth surface, wherein the group III-nitride film having anN-face is smoother than an N-face group III-nitride film grown on asubstrate without a misorientation angle.
 22. A method for enhancingcharge transport properties of a nitride device, comprising: fabricatingthe nitride device using N-face nitride layers grown on a substratehaving a growth surface with a misorientation angle between 0.5 and 10degrees relative to a miller indexed crystallographic plane [h, i, k, l]of the substrate, where h, i, k, l are miller indices; and aligning achannel of the nitride device substantially perpendicular to amisorientation direction of the misoriented N-face (Al,Ga,In)N layergrown on a misoriented substrate, wherein charge transport propertiesare enhanced in the misorientation direction.